1. Field of the Invention
The present invention relates to a signal switching circuit, for example a signal switching circuit suitable for use in a satellite broadcast receiving outdoor converter in which satellite broadcast signals of counterclockwise and clockwise circularly polarized waves are outputted arbitrarily to two output terminals.
2. Description of the Related Art
A conventional signal switching circuit will now be described with reference to FIGS. 5 and 6. First, FIG. 5 illustrates a block configuration, in which a first signal, say, a satellite broadcast signal of a counterclockwise circularly polarized wave, is inputted to a first input terminal 51 and a second signal, say, a satellite broadcast signal of a clockwise circularly polarized wave, is inputted to a second input terminal 52. These signals are inputted from a down converter (not shown).
The first signal is inputted through the first input terminal 51 to both a first terminal 55a of a first switch circuit 55 and a first terminal 56a of a second switch circuit 56, while the second signal is inputted through the second input terminal 52 to both a second terminal 55b of the first switch circuit 55 and a second terminal 56b of the second switch circuit 56. One of the two signals is selected in each of the switch circuits 55 and 56 and the selected signals are outputted respectively to third terminals 55c and 56c of the switch circuits 55 and 56 and are then outputted respectively to a first output terminal 53 and a second output terminal 54.
The first and second switch circuits 55 and 56 are of the same configuration, which is illustrated in FIG. 6 (only the first switch circuit 55 is shown). Two diodes 61 and 62, which are connected together in series, are disposed between the first and third terminals 55a, 55c in such a manner that their cathodes and anodes are located on the first terminal 55a side and the third terminal 55c side, respectively. A connection point between the two diodes 61 and 62 is connected to the anode of a diode 64 through a DC cut-off capacitor 63, the cathode of the diode 64 being grounded high-frequencywise by means of a DC cut-off capacitor 65.
Two diodes 66 and 67, which are connected together in series, are disposed between the second and third terminals 55b, 55c in such a manner that their cathodes and anodes lie on the second terminal 55b side and the third terminal 55c side, respectively. A connection point between the two diodes 66 and 67 is connected to the cathode of a diode 69 through a DC cut-off capacitor 68, the anode of the diode 69 being grounded high-frequencywise by means of the DC cut-off capacitor 65.
Voltage E is applied to the anodes of the diodes 62 and 67 through a resistor 70. The cathode of the diode 61, the anode of the diode 64, and the cathode of the diode 69 are connected to a first voltage application terminal 74 through resistors 71, 72, and 73, respectively. The cathode of the diode 66 and a connection point between the diodes 64 and 69 are connected to a second voltage application terminal 77 through resistors 75 and 76, respectively. The voltage E is applied to the first and second voltage application terminals 74, 77 while switching polarity by means of a change-over switch 78 of a two-circuit two-contact configuration.
When the voltage E is applied to the first voltage application terminal 74 and the second voltage application terminal 77 is grounded, the diodes 61 and 62 turn OFF and the diode 64 turns ON. Further, the diodes 66 and 67 turn ON and the diode 69 turns OFF. As a result, the second signal which has been inputted to the second terminal 55b is outputted to the third terminal 55c. At this time, the first signal which has been inputted to the first terminal 55a is not outputted.
When the first voltage application terminal 74 is grounded and the voltage E is applied to the second voltage application terminal 77, the diodes 61 and 62 turn ON and the diode 64 turns OFF. Further, the diodes 66 and 67 turn OFF and the diode 69 turns ON. As a result, the first signal which has been inputted to the first terminal 55a is outputted to the third terminal 55c. At this time, the second signal which has been inputted to the second terminal 55b is not outputted.
Thus, the first signal inputted to the first input terminal 51 or the second signal inputted to the second input terminal 52 is outputted to the first output terminal 53 by the first switch circuit 55.
Likewise, the first signal inputted to the first input terminal 51 or the second signal inputted to the second input terminal 52 is outputted to the second output terminal 54 by the second switch circuit 56.
In the conventional configuration described above there are used six diodes for outputting the first or the second signal to the first output terminal 53. Likewise, the same number of diodes are used for outputting the first or the second signal to the second output terminal 54. As a result, not only the number of diodes used increases, but also the number of parts and wires for turning ON or OFF the diodes increases, with consequent complication of construction and difficulty in attaining the reduction of size.
The present invention has been accomplished in view of the above-mentioned points and it is an object of the invention to provide a signal switching circuit which permits the reduction in number of diodes used, thereby attaining the simplification of construction and the reduction of size.
For achieving the above-mentioned object, in a first aspect of the present invention there is provided a signal switching circuit comprising a first input terminal to which a first signal is inputted, a second input terminal to which a second signal is inputted, first and second output terminals from which the first or the second signal is outputted, first to fourth diode pairs each comprising two diodes which are connected together in series, the first to fourth diode pairs being disposed respectively between the first input terminal and the first output terminal, between the second input terminal and the first output terminal, between the first input terminal and the second output terminal, and between the second input terminal and the second output terminal, and first to fourth capacitor means connected respectively between diode-to-diode connection points in the respective diode pairs and the ground, wherein the first or the second diode pair is rendered conductive in an alternative manner, the third or the fourth diode pair is rendered conductive in an alternative manner, and a low pass filter for passing therethrough the corresponding first or second signal is constructed by residual inductance in each of the diode pairs thus rendered conductive and the associated capacitor means.
In a second aspect of the present invention there is provided, in combination with the above first aspect, a signal switching circuit wherein the first to fourth capacitor means are constructed by first to fourth strip lines respectively, one ends of the strip lines being connected respectively to the diode-to-diode connection points in the diode pairs, with the opposite ends thereof being made free, the length of each of the first and third strip lines is set shorter than a quarter wavelength of the first signal, and the length of each of the second and fourth strip lines is set shorter than a quarter wavelength of the second signal.
In a third aspect of the present invention there is provided, in combination with the above second aspect, a signal switching circuit wherein the two diodes in each of the diode pairs are connected together so that the directions of respective PN junctions are the same.